As illustrated in U.S. Pat. No. 6,512,487 entitled Wide Band Phased Array Antenna and Associated Methods; U.S. Pat. No. 6,771,221 entitled Enhanced Bandwidth Dual Layer Current Sheet Antenna; U.S. Pat. No. 7,084,827 entitled Phased Array Antenna with an Impedance Matching Layer and Associated Methods; as well as U.S. Pat. No. 6,552,687 entitled Enhanced Bandwidth Single Layer Current Sheet Antenna, arrays of closely or tightly coupled dipole arrays are described. These inventions are based on an invention by Benedict A. Munk described in U.S. Pat. No. 4,125,841 entitled Space Filter. It is reported that it was Munk's invention to add a coupling element at the end of each half wavelength dipole to allow the phased array to be exceedingly broadbanded.
It is noted that the dipole itself is capable of an octave bandwidth, whereas derivative antennas approach a decade of bandwidth assuming the appropriate kind of coupling design between the dipoles. Moreover, planar two dimensional arrays of a sheet of dipoles increase gain or directivity; and by adding coupling in orthogonal directions one can also achieve multiple polarizations for the phased array.
Applications for such planar phased arrays are in general for broadband surveillance, electronic warfare applications and any applications which require very broadband phased arrays.
When utilizing such closely coupled dipole arrays for transmit/receive operations, it is common to provide either a circulator or a double pole, double-throw transmit/receive switch at each of the feeds of the dipoles in order to isolate the transmitter from the receiver and vice versa. The circulators and transmit/receive switches are in general referred to as duplexers. However, when it is intended for these antennas to be driven in the transmit and receive modes alternately, placing a circulator or transmit/receive switch at each of the antenna feeds for the dipoles can be physically impossible, depending on frequency of operation, due to the limitations of the physical size of such circulators and switches which precludes their use above the ground plane normally used for such planar arrays.
For instance, circulators tend to be too large at the frequencies of interest. This is because the spacing between the electronics is approximately one half wavelength at the operating frequency. Note that at the highest frequency for which the antenna will operate, the spacing between the elements needs to be no more than one half wavelength at this frequency. Duplexers in the form of circulators and T/R switches are much too large to be placed at the feedpoint of a dipole, especially when these duplexing units are above the ground plane for the planar array. Moreover, the typical circulators are bandwidth-limited and TR switches have excessive losses. Thus T/R switches absorb power during the transmission process and limit sensitivity on the receive side. It will be appreciated that for a decade bandwidth switch there could be as much as a dB loss or even more if high power switches are used. Note also that any piece of electronics that is interposed between the receiver or transmitter and the antenna will have parasitics that will limit the bandwidth.
In summary, circulators have limited bandwidth, limited usually to an octave. Moreover, circulators get bulkier and lossier as one seeks to achieve a 5:1 bandwidth. Thus, using a circulator limits the bandwidth performance. On the other hand, transmit/receive switches with pin diodes result in unacceptable losses that limit performance. Moreover, dipoles require balanced inputs and the use of baluns to convert an unbalanced line to a balanced line is undesirable due to the added parasitics and losses.
Two other factors which further complicate phased array implementations of circulators include the use of high field strength bias magnets which must be shielded to prevent interaction with the shielding significantly adding to the bulk of the structure.
Finally as mentioned above, balanced lines require baluns which are differential single ended to balanced devices required between the feed and the circulator, or the feed and the transmit receive switch. The use of baluns adds additional circuitry which further degrades performance in terms of loss, match and bandwidth.
Such weight and size limitations as well as limitations on performance are particularly acute when, for instance, planar arrays of miniature dipoles exceed 1,000×1,000 dipole arrays or greater. While it is theoretically possible to locate the duplexing circuitry beneath the ground plane of the antenna, it is highly desirable to be able to eliminate duplexers so as to be able to fabricate reasonable size and planar arrays, with the antenna elements existing above the ground plane. In short, there is a need to eliminate the large amount of electronics directly connected at the feed of these antennas when contemplating transmit/receive functions.